Now that miniature liquid crystal displays are readily available, a variety of devices using such displays have become popular. One example system that has become quite popular worldwide is Nintendo's GAME BOY COLOR® handheld video game system. The LCD Screen of GAME BOY COLOR® can display a total of 32,768 colors. However, the internal hardware that drives the GAME BOY COLOR® liquid crystal display has a much more limited color resolution in terms of the number of different colors that can be displayed simultaneously on the liquid crystal display screen.
Specifically, the GAME BOY COLOR® system is character-mapped rather than bit-mapped, and uses a color palette-based color-mapping arrangement to display the different colors of background and moving object video game characters. The internal liquid crystal display driver hardware is limited as to the number of color palettes that can be active at any one time. This has the effect of limiting the number of colors that may be displayed simultaneously on the LCD screen. For example, even though the color LCD display is capable of displaying more than 32,000 different colors, internal hardware limits the number of different colors to a maximum of 56 different colors at any particular instant in time.
This color mapping functionality of GAME BOY COLOR® provides advantages in terms of lower memory requirements (and thus lower cost) as compared systems with systems using a full-color frame buffer to allow the color of each individual display pixel to be independently specified. This trade-off is quite acceptable for fast-paced high-action video game play where color richness is not as important as color repertoire. However, for the display of photographic-quality still pictures, it would be highly desirable to achieve greater color diversity closer to what might be achieved with a full color frame buffer.
In order to display more colors on the LCD screen, we need to work around the limitation of the display system and simultaneously display as many different colors as possible. We have developed an invention to solve this problem that can be implemented on the GAME BOY COLOR® system but could be applied to any low-cost LCD display device with hardware that limits the number of simultaneously-displayable colors to less than the total number of colors the display device is capable of.
In accordance with one aspect of our invention, we display more colors by changing the color palette line by line during active display time. Such color palette updates can be accomplished by taking advantage of the horizontal blanking interval between rasterization of successive lines on the display. During each horizontal blanking period, we can rewrite half of the color palettes loaded into the active memory area. This means that we can rewrite all of the color palettes for each pair of display lines—providing a much larger total number of colors that may be simultaneously displayed on the LCD display.
In accordance with a further aspect of the present invention, we can optimize the conversion of full color bitmapped source images to color mapped images in a way that takes maximal advantage of the color mapping updates described above. For example, we can use an image subdivision process that breaks the source image up into optimal chunks corresponding to the association between color mapping data and portions of the image to be displayed. We can also use a particular subset of the display area provided by the LCD display to optimize such correspondence. A pixel averaging data-reduction technique using a closest-color color-reduction method based on Euclidean distance in 3D color space can be used to quantize the colors for the color map.
In further detail, we can convert a full-color source image into a color-mapped image suitable for display on the LCD display system using techniques that are optimized for the color palette updates described above. For example, we convert from a source image to a target image based on an image subdivision process that breaks the source image up into optimal chunks relating to the association between color palettes and image portions. We also choose to display our images within a square subset of the display area provided by the LCD display—again in order to optimize correspondence between particular image portions and color palettes. As a result, we can display a color image with very high color resolution (e.g., having as many as 2048 different colors) on hardware intended to permit simultaneous display of a much smaller number of different colors (e.g., only 56 different colors simultaneously).
In accordance with a further aspect of the invention, we use a pixel averaging data-reduction technique to convert a full color bitmapped source image into a color mapped image suitable for display on the limited-resource portable LCD display system. We use a closest-color color-reduction method based on Euclidean distance in 3D color space to pick the optimal subset of colors that results from averaging four neighboring pixel color values to provide a single averaged color. We can also use color distance to determine which of four selected palette colors we will assign to particular source image pixels. In particular, the preferred embodiment gets four colors from each 2-by-2 pixel minitile, and averages these four RGB value to get one color to represent that 2-pixel by 2-pixel minitile. This yields eight colors within a 16-pixel by 2-pixel tile. The preferred embodiment then uses a 3D color-distances calculation to get four colors out of the eight colors as a palette to represent that 16-pixel by 2-pixel tile. Once the four-color palette is obtained, the preferred embodiment uses the 3D distance calculation to reproduce the pixels using one of the four colors in that certain tile.